The present invention relates generally to the field of distributed systems and more particularly to the precision of a time synchronization in a distributed system that uses the IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems (IEEE 1588-2008).
IEEE 1588-2008, also known as Precision Time Protocol (PTP), is a protocol standard to synchronize real-time clocks in the nodes of a distributed system that communicate using a network. This standard defines a protocol that enables precise synchronization of clocks in measurement and control systems implemented with technologies such as network communication, local computing, and distributed objects. The protocol is applicable to systems communicating by local area networks supporting multicast messaging including, but not limited to, Ethernet. The protocol enables heterogeneous systems that include clocks of various inherent precision, resolution, and stability to synchronize to a master clock. The protocol supports system-wide synchronization accuracy in the sub-microsecond range with minimal network and local clock computing resources. The standard permits synchronization accuracies to better than 1 ns. IEEE 1588-2008 is primarily designed for local systems requiring accuracies beyond those attainable using Network Time Protocol which can achieve an accuracy from one millisecond to tens of milliseconds. It is also appropriate for applications that cannot bear the cost of a GPS receiver at each synchronized node, or for which GPS signals are not accessible.
Ethernet structures and protocols are continually enhanced to accommodate the need for ever-faster communications and increasingly accurate synchronization in distributed systems. For example, fiber optic communications, which enables fast, high bandwidth, communication over long distances via modulated light waves carried in nearly transparent fibers, have been incorporated into Ethernet-based communication links. Often a fiber optic Ethernet connection will accommodate four or more “optical lanes” of communication simultaneously. Four or more frequencies can be used simultaneously in one optical fiber to carry four or more data streams. These optical lanes are often the conduits for information from many more “electrical lanes”, of communication over the Ethernet connection. The electrical lanes can be shared by “virtual lanes” of communication. A few optical lanes are shared by many more virtual lanes. The number of virtual lanes is usually the least common multiple of the number of electrical lanes and the number of optical lanes. Data being transferred on a specific virtual lane is assigned to one specific optical lane, but that optical lane is shared (multiplexed) among many virtual lanes so that the data on an optical lane (from a given virtual lane) is usually interspersed with data from other virtual lanes on that same optical lane.
Measurement and control applications, which require accurate timing and synchronization, are increasingly employing distributed system technologies such as network communication, local computing, and distributed objects. Many of these applications are enhanced by having an accurate system-wide sense of time achieved by having local clocks in each sensor, actuator, or other system device. The improvement of the accuracy and granularity (smallest visible time duration) of distributed clocks in such systems is an active area of research.